Java code to find diameter of a binary tree

Diameter of a binary tree is the maximum width at any node level.
The maximum width can be at last level and that can be the diameter of a binary tree.

You can watch me code this solution on youtube.

We need three values to find the diameter of a binary tree.

1. diameter of left-subtree
2. diameter of right-subtree
3. sum of height of left-subtree and right sub-tree and plus one for the root node

Which ever is the maximum value among these three values is diameter of the tree at that node.

Code to find the diameter of a binary tree.

private static int diameter(final BinaryTreeNode root){
        if(root == null) return 0;

    final int heightOfTheLeftSubtree = height(root.left);
    final int heightOfTheRightSubtree = height(root.right);

    final int diameterThroughTheRoot = heightOfTheRightSubtree + heightOfTheLeftSubtree + 1;

    final int diameterOfTheLeftSubtree = diameter(root.left);
    final int diameterOfTheRightSubtree = diameter(root.right);

    return Math.max(diameterThroughTheRoot, Math.max(diameterOfTheRightSubtree, diameterOfTheLeftSubtree));
    }

You can see in the above code that, I am getting the max value among the three values.

You can read my code on github, for better reading experience.

Complete Code:-

public class DiameterOfABinaryTree{

    private static int height(final BinaryTreeNode root){
        return root == null ? 0 : 1 + Math.max(height(root.left), height(root.right));
    }

    private static void printBinaryTreeNode(final BinaryTreeNode node){
        if(node == null) System.out.println("NULL Node");

    System.out.format("[%d] ", node.data);
    }
    private static class BinaryTreeNode{
        private int data;
        private BinaryTreeNode left;
        private BinaryTreeNode right;
    
        private BinaryTreeNode(final int data, final BinaryTreeNode left, final BinaryTreeNode right){
            this.data  = data;
    	this.left  = left;
    	this.right = right;
        }
        private BinaryTreeNode(final int data){
            this(data, null, null);
        }
    }
    
    public static void main(final String[] args){
        final BinaryTreeNode one   = new BinaryTreeNode(1);
        final BinaryTreeNode two   = new BinaryTreeNode(2);
        final BinaryTreeNode three = new BinaryTreeNode(3);
        final BinaryTreeNode four  = new BinaryTreeNode(4);
        final BinaryTreeNode five  = new BinaryTreeNode(5);
        final BinaryTreeNode six   = new BinaryTreeNode(6);
    
        four.left = one;
        four.right = five;
    
        five.right = six;
    
        one.left = two;
        one.right = three;

    System.out.format("Diameter of the binary tree is %d.%n", diameter(four));
    }

    private static int diameter(final BinaryTreeNode root){
        if(root == null) return 0;

    final int heightOfTheLeftSubtree = height(root.left);
    final int heightOfTheRightSubtree = height(root.right);

    final int diameterThroughTheRoot = heightOfTheRightSubtree + heightOfTheLeftSubtree + 1;

    final int diameterOfTheLeftSubtree = diameter(root.left);
    final int diameterOfTheRightSubtree = diameter(root.right);

    return Math.max(diameterThroughTheRoot, Math.max(diameterOfTheRightSubtree, diameterOfTheLeftSubtree));
    }
}